How often are your cells replaced?

Your body is constantly replacing old cells with new ones at the rate of millions per second. By the time you finish reading this sentence, 50 million of your cells will have died and been replaced by others. Some are lost through 'wear and tear', some just reach the end of their life, and others deliberately self-destruct. The life cycle of every cell is carefully controlled, so you should always have just the right number of each type of cell.